But, however in that regard it is interesting to see the shoe on the other foot for Firefox this time around.
Historically Firefox used to rely on remembering recent intermediates it had seen to "fill in the blanks" when trying to access a site that doesn't bother providing any reason to trust the certificate it offers†.
But, Mozilla spent years collecting knowledge of all unconstrained intermediates for trusted root CAs. Then they shipped the entire set as part of Firefox. So, a modern Firefox visiting a site that only has a Let's Encrypt certificate and no other reason to be trusted, sees that and goes oh, that's issued by Let's Encrypt's R3, which I have a copy of, and so it's trustworthy. Done.
Whereas for several other popular browsers they're looking at potentially stale local information, and may conclude that they can't trust this site because some stale data has expired or they rely on an already expired certificate sent to them in some cases rather than disregarding it.
† You're supposed to provide a "chain" (it doesn't actually need to be a chain, but that's most compatible) of other certificates to show why the leaf certificate you have is trustworthy, e.g. my certificate is from Z9, Z9 has a certificate from Big Trusted Corp Q46, Big Trusted Corp Q46 has a certificate from Very Famous Trusted CA, and the relying party (well, their client software) goes, "Oh, I see, and I trust Very Famous Trusted CA, so that means I trust you're you". But, lots of web sites (maybe 10-20% and more of the smallest with negligible IT budget) don't get this correct so web browsers try to work around it.